Applicant's U.S. Pat. No. 4,928,030 teaches two- and three-axis piezoelectric actuators that position an object such as a rod or motor shaft by walking traction, A lifter piezoelectric actuator portion positions the actuator's traction member perpendicular to the object's surface. A tangenter piezoelectric actuator portion positions the actuator's traction member tangential to the object's surface. Lifter and tangenter portions of an actuator are integrally constructed and independently electrically controllable. Cyclical coordinated lifter and tangenter actions can cause the traction member to walk the object's surface in a smooth walking manner. A smooth walking cycle operation consists of activating the lifter to apply a predetermined normal force between the traction member and the object while the tangenter translates the traction member at a speed equal to the surface speed of the object. During smooth walking application and removal of normal force, no mechanical work is done by the traction member on the object. As the normal force is applied, a tangential strain is added to the tangenter portion. The product of the tangential force and the tangential distance traveled during the power stroke portion is the work done on the object. At the end of the power portion of the cycle the tangential strain is removed as the normal force is removed by the lifter, still maintaining zero relative speed between object and traction member. After the traction member leaves the object's surface, the traction member retraces, that is, it reverses tangential stroke direction and changes speed until the opposite extreme tangential position is reached, thereby preparing for a new stroke. This is a smooth walking cycle because sliding is avoided.
Actuators execute walking cycles in pairs, one actuator performing a power stroke while the other retraces. A predetermined coordinated positioning of the traction members of both actuators results in smooth walking. Smooth walking as defined in applicant's U.S. Pat. No. 4,928,030 is uninterrupted and smooth tractional power transmission without sliding.
The piezoelectric materials described in U.S. Pat. No. 4,928,030 are generally electrically polarized ferroelectric ceramics, This class of materials is relatively brittle, having relatively little tensile strength. In addition, the temperatures above and below the usual room temperature at which electrical polarization is irreversibly lost, usually called the Curie temperatures, are relatively low. These physical properties are a detriment in some applications of walking actuators. U.S. Pat. No. 4,928,030 also teaches the use of relatively high applied voltages to achieve desirably large mechanical strokes. High voltages are a disadvantage in the context of solid state electronic drive devices, such devices having evinced more efficient operation with low voltages with relatively large currents.
Applicant's copending patent application Ser. No. 07/488,548 filed Mar. 5, 1990 teaches the use of Fourier generation of non-sinusoidal mechanical wave forms needed for smooth walking. The teachings are primarily directed toward piezoelectric actuators, but are also directed toward actuators that function in a manner similar to piezoelectric ones but use other known actuator forces. This method allows partial compensation for nonlinear transducer action by operations in the time domain, for example, adjusting phases of stimuli.
Applicant's present invention is to replace the electrodeformable tangenter and lifter layers of the prior art with thermoexpandable layers of lifters and tangenters and to emulate their positioning capabilities. The thermoexpandable actuators use mechanical Fourier addition of the layers for positioning the actuators.